A conventional tire is a torus-shaped structure that is intended to be mounted on a rim, pressurized by an inflation gas and squashed down onto the ground under the action of a load. At any point on its tread surface, which is intended to come into contact with the ground, the tire has a double curvature: a circumferential curvature and a meridian curvature. Circumferential curvature means a curvature in a circumferential plane, defined by a circumferential direction, tangent to the tread surface of the tire in the rolling direction of the tire, and a radial direction, perpendicular to the axis of rotation of the tire. Meridian curvature means a curvature in a meridian or radial plane, defined by an axial direction, parallel to the axis of rotation of the tire, and a radial direction, perpendicular to the axis of rotation of the tire.
In the following text, the expression “radially inner or, respectively, radially outer” means “closer to or, respectively, further away from the axis of rotation of the tire”. The expression “axially inner or, respectively, axially outer” means “closer to or, respectively, further away from the equatorial plane of the tire”, the equatorial plane of the tire being the plane that passes through the middle of the tread surface of the tire and is perpendicular to the axis of rotation of the tire.
It is known that the flattening of the tire on horizontal ground, in a circumferential plane and in a meridian plane, is conditioned by the values of the circumferential and meridian radii of curvature, respectively, at the points of the tread surface that are positioned at the limits of the contact patch in which the tire is in contact with the ground. This flattening is all the easier the larger these radii of curvature are, that is to say when the curvatures are low, since the curvature at any one point, in the mathematical sense, is the inverse of the radius of curvature. It is also known that the flattening of the tire has an impact on the performance of the tire, in particular rolling resistance, grip, wear and noise.
Consequently, those skilled in the art, tire specialists, seeking to obtain a good compromise between the expected performance of the tire, such as wear, grip, endurance, rolling resistance and noise, this list not being exhaustive, have developed alternative solutions to conventional tires in order to optimize the flattening thereof.
A conventional prior art tire generally has a high meridian curvature, that is to say a small meridian radius of curvature, at the axial ends of the tread, known as shoulders, when the tire, mounted on its mounting rim and inflated to its recommended use pressure, is subjected to its service load. The mounting rim, the use pressure and the service load are defined by standards, such as the standards of the European Tire and Rim Technical Organization (ETRTO), for example. A conventional tire bears the load applied, substantially by the axial ends of the tread, or shoulders, and by the sidewalls connecting the tread to beads that ensure the mechanical connection of the tire to its mounting rim. It is known that meridian flattening of a conventional tire, with a low meridian curvature at the shoulders, is generally difficult to obtain.
The document U.S. Pat. No. 4,235,270 describes a tire having an annular body made of elastomeric material, comprising a radially outer cylindrical part, at the periphery of the tire, that can comprise a tread, and a radially inner cylindrical part that is intended to be mounted on a rim. A plurality of walls that are spaced apart in the circumferential direction extend from the radially inner cylindrical part to the radially outer cylindrical part and bear the load. Moreover, sidewalls can connect the radially inner cylindrical part and the radially outer cylindrical part in order to form, in association with the tread and the sidewalls, a closed cavity and thereby to allow the tire to be pressurized. However, such a tire has a high mass compared with a conventional tire and, on account of its massive nature, is likely to dissipate a large amount of energy, which can limit its endurance and thus its useful life.
The document WO 2009087291 describes a tire structure comprising an internal, or radially inner, annular shell and an external, or radially outer, annular shell that are connected by two sidewalls and by a bearing structure. According to that invention, the bearing structure is pressurized and divides the annular volume of the tire into a plurality of compartments or cells, and the sidewalls are connected to or integrated with the bearing structure. In that case, the load applied is borne both by the bearing structure and the sidewalls. The distribution of pressure in the contact patch is not homogeneous across the axial width of the contact patch, with raised pressures at the shoulders on account of the difficulty of meridian flattening because of the connection between the sidewalls and the bearing structure. These raised pressures at the shoulders are likely to generate significant wear at the shoulders of the tread.
The document WO 2005007422 describes a compliant wheel comprising a compliant band and a plurality of spokes extending radially inwards from the compliant band to a hub. The compliant band is intended to adapt to the area of contact with the ground and to envelop obstacles. The spokes transmit the load borne between the compliant band and the hub, by virtue of the spokes that are not in contact with the ground being tensioned. Such a compliant wheel requires optimization of the distribution of the spokes so as to ensure a substantially cylindrical periphery. Moreover, a compliant wheel has a relatively high mass compared with a conventional tire.